(1) Field of the Invention
The present invention relates to a fluorine resin composition having a high light transmission and excellent heat resistance and solvent resistance, which is suitable as a material for making an optical fiber, an optical lens and a film.
(2) Description of the Related Art
A fluorine polymer generally has an excellent heat resistance, solvent resistance and water resistance, and polytetrafluoroethylene and polyvinylidene fluoride are provided as industrial products and are utilized in various fields.
Nevertheless, these heretofore developed fluorine polymers are crystalline and the light transmission is poor, and therefore, it is difficult to use these polymers as optical materials for which a high light transmission is required.
A perfluoroalkyl methacrylate polymer is known as a fluorine polymer having an excellent light transmission, and although this polymer is used as an optical fiber cladding, since the heat resistance and solvent resistance of this polymer are poor, this polymer is not suitable as a material having an excellent environmental resistance.
Japanese Unexamined Patent Publication No. 63-18964 discloses an amorphous copolymer comprising the following units [A] and [C]: ##STR2## and EQU --CF.sub.2 --CF.sub.2 -- [C]
and it is taught that this amorphous copolymer has a high light transmission and an excellent heat resistance and solvent resistance.
U.S. Pat. No. 4,754,009 discloses a copolymer of perfluoro-2,2-dimethyl-1,3-dioxole represented by formula [A] with other polymerizable fluorine-containing monomer [B], and it is taught that the glass transition temperature of the copolymer is not lower than 140.degree. C., and therefore, it is considered that the copolymer has an excellent heat resistance.
Nevertheless, in this copolymer of perfluoro-2,2-dimethyl-1,3-dioxole [A] with other copolymerizable monomer [B], if the copolymerization amount of units [A] is smaller than 30 mole %, the glass transition temperature of the copolymer is lowered and the heat resistance becomes poor, and simultaneously, a partial crystallinity is manifested and the light transmission is lowered.
In the copolymer in which the copolymerization amount of units [A] is larger than 40 mole %, the glass transition temperature is high, and the copolymer is amorphous and has a good light transmission, but since the copolymer has a rigid structure, a film prepared from this copolymer has a low elongation at break and is readily broken.
A copolymer composed of 89 mole % of perfluoro-2,2-dimethyl-1,3-dioxole units [A] and 11 mole % of tetrafluoroethylene units [C] has a high heat resistance represented by a glass transition temperature of 250.degree. C., but the elongation at break of the film is as low as 4.3%.
In Japanese Unexamined Patent Publication No. 63-18964, it is taught that the glass transition temperature of a copolymer composed of 56.9 mole % of units [A] and 43.1 mole % of tetrafluoroethylene units [C] is 119.degree. C., but the elongation at break of a film obtained from this copolymer is as low as 4.4%, and that the glass transition temperature of a copolymer composed of 36.6 mole % of units [A] and 63.4 mole % of tetrafluoroethylene units [C] is 90.degree. C. and the elongation at break of a film prepared from this copolymer is as low as 4.1%. On the other hand, the glass transition temperature of a copolymer composed of 22.1 mole % of units [A] and 77.9 mole % of tetrafluoroethylene units [C] is 73.degree. C., but the elongation at break of a film prepared from this copolymer is as high as 58.2%. Further, the heat resistance of the film is not satisfactory.
A copolymer comprising at least 30 mole % of perfluoro-2,2-dimethyl-1,3-dioxole units [A] is an amorphous polymer having a good light transmission and a high heat resistance, but the elongation at break of a film prepared from this copolymer is as low as 7% or lower, and therefore, the handling property of this copolymer is poor and it is difficult to use this copolymer as a film, an optical material or an optical fiber cladding.